Diene polymers, including SBR, are well known and various methods of preparing such polymers are also well known in the art. Such rubbers have many applications, especially in tires, because of their excellent wear resistance, low rolling resistance and excellent wet skid resistance without adversely affecting the desirable physical properties of the polymer. For automobile tire applications, rubbers that have low hysteresis loss are especially important for tread applications.
Diene polymers, including SBR, in which the low hysteresis loss property is particularly good have been obtained in the past using an organolithium compound as the initiator in a hydrocarbon solvent polymerization and thereafter coupling a tin compound to the end of the polymer replacing the lithium (Japanese Patent Application laid open No. 1982-55912 and others). According to this method, when a halogenated tin compound is added to the polymerization mixture, the polymerization is deactivated and the molecular structure of the polymer obtained is fixed at the time of the addition of the tin compound. Therefore, in such a method the tin compound is generally added at the end of the polymerization, after the desired molecular weight of the polymer has been reached. Thus, a batch process must be used since the remaining unreacted monomers must be removed.
A more economical continuous polymerization process was disclosed in Japanese patent application laid open No.1988-235305 where a tin compound, such as tin tetrachloride, is added approximately mid-way in the copolymerization of butadiene and styrene. The portion of the polymer chains that forms a tin-carbon linkage will not propagate any longer but the remaining polymer chains that still contain the active lithium end will continue the polymerization. The result of such a process is the formation in the polymerization reaction mixture of a polymer blend containing the higher molecular weight polymers as well as the lower molecular weight polymers. The advantage of this method is the formation of a blend without the need for using separate blending step. The disadvantage of the continuous polymerization process is that a wide molecular weight distribution of the polymer is obtained and such polymers as a rule have inferior hysteresis property. In this respect the batch process is preferred because there is a much better control of polymerization conditions making it possible to obtain diene polymers with a much narrower molecular weight distribution. Such polymers possess a better hysteresis property. The disadvantage of the batch process is that it is less economical.
The above-described methods of preparing diene/vinyl monomer copolymers did not yield polymers which contain a proper balance between the wear resistance, rolling resistance and wet skid property because such polymers did not have the proper balance between the amount of the styrene and the butadiene and the proper balance between the various molecular weights of the chains. When the vinyl content of the butadiene part of styrene-butadiene copolymer in the rubber composition is increased, the fracture properties and the wear resistance is decreased but the wet skid property of the tire is improved. Thus, optimizing the content of the bound styrene, the microstructure of the butadiene part and the molecular weight distribution are insufficient to obtain a rubber composition that can satisfy the various requirements of low rolling resistance (i.e. low hysteresis loss) and good wet traction that are important for tire tread.
To obtain such a desired balance of properties U.S. Pat. No. 5,587,420 discloses a method of polymerizing a conjugated diene monomer or a mixture of a conjugated diene monomer and a vinyl aromatic hydrocarbon monomer in a hydrocarbon solvent by using an organolithium compound as the initiator with the addition of an organotin compound, including halogenated tin compounds, to the polymerization system in a period of growth of the polymer chain between a time immediately after the start of the polymerization to a time before the end of the polymerization. It is important to note that the tin compounds are added to the polymerization reaction only once and the disclosed method is a batch process.